1. Field of the Invention
This invention relates to voltage amplifiers, and more particularly to voltage amplifiers intended to drive high output capacitive loads with rail-to-rail amplification.
2. Description of the Related Art
With conventional voltage amplifiers such as operational amplifiers having dominant pole compensation, at least a 90xc2x0 phase shift is produced in the output signal at all but the lowest frequencies, due to the use of an internal compensation capacitor. (The term xe2x80x9cphase shiftxe2x80x9d as used herein refers to the shift from the ideal output signal phase. Since an op amp with a feedback loop connected to its inverting input is intended to produce a 180xc2x0 phase shift between the input and output, the term xe2x80x9c180xc2x0 phase shiftxe2x80x9d as used herein thus refers to the input signal actually being in-phase with the output signal.) Such an operational amplifier (op amp) circuit is illustrated in simplified form in FIG. 1. The op amp 2 includes non-inverting 4 and inverting 6 inputs, with its output resistance represented by internal resistor Ro. It is operated off positive and negative reference voltage terminals V+ and Vxe2x88x92 such as 5 volts and ground. An input signal Vin is applied to the non-inverting input terminal 4, while a feedback resistor Rfb is connected between its output terminal 10 and the inverting input 6. A gain resistor Rgain is typically connected between the inverting input and ground, establishing the magnitude of the output signal Vo in accordance with the ratio between Rfb and Rgain.
The load connected to the output terminal 10 typically includes a resistive component Rl and a capacitive component Cl. The latter impedance typically includes stray capacitive loads associated with printed circuit traces, connectors, coaxial cable, etc.
For practical amplifiers, the phase shift at the output is typically considerably more than 90xc2x0, due in part to the op amp""s output resistance. Also, some op amps appear inductive at the output, causing the stray capacitive load to further contribute to output phase shift. If the total phase shift is 180xc2x0 or more when the loop gain is greater than unity, the amplifier""s negative feedback becomes positive and oscillation results.
While the op amp may be intended for relatively low frequency inputs, such as signals up to about 100 KHz, the oscillation can be at many different frequencies, including high frequencies of 500 KHz or more, depending upon numerous factors such as the amount of load capacitance applied to the circuit. The purpose of the compensation capacitor Ccomp is to roll off the amplifier gain to below unity before the phase shift exceeds 180xc2x0, but the additional sources of phase shift discussed above can frustrate this goal and still lead to oscillation.
One prior attempt to prevent such oscillation is illustrated in FIG. 2, which adds a blocking resistor Rbl between the output of amplifier 2 and the load impedance.
While the addition of a blocking resistor has been found to successfully reduce the output phase shift, at the same time it increases the output resistance and thereby reduces the range of the output voltage swing to noticeably less than the difference between V+ and Vxe2x88x92, which is commonly called the xe2x80x9crail-to-railxe2x80x9d range. In another prior circuit, illustrated in FIG. 3, the connection for the feedback resistor Rfb is moved to the opposite side of the blocking resistor Rbl from the output of op amp 2, and a feedback capacitor Cfb is connected between the op amp output and the inverting input 6. With this circuit the feedback capacitor Cfb presents a high impedance or open circuit at low frequencies, placing the blocking resistor Rbl inside the feedback loop in series with feedback resistor Rfb. At high frequencies, on the other hand, Cfb presents a significantly lower impedance than Rfb, establishing the primary feedback path through Cfb and effectively placing Rbl outside the feedback loop, where it can still reduce the phase shift due to the load capacitance Cl. Cfb reduces the gain at medium to high frequencies, but this effect can be reduced by adding some resistance in series with Cfb.
While Rbl does not reduce the output phase shift due to Cl for low frequencies at which it is effectively inside the amplifier feedback loop, the phase shift due to Cl is small for low frequencies. Rbl does not increase the output resistance at low frequencies, but its effective series connection with Rl in the low frequency range results in a voltage drop across Rbl that reduces the voltage swing for Vo to a range significantly less than the rail-to-rail range at high load currents.
An op amp with a conventional CMOS output stage that yields a rail-to-rail output voltage swing is illustrated in FIG. 4. This general type of circuit is described in Wu et al., xe2x80x9cDigital-Compatible High Performance Operational Amplifier with Rail-to-Rail Input and Output Rangesxe2x80x9d, IEEE Journal of Solid-State Circuits, Vol. 29, No. 1, January 1994, pages 63-66, and Monticelli, xe2x80x9cA Quad CMOS Single-Supply Op Amp with Rail-to-Rail Output Swingxe2x80x9d, IEEE Journal of Solid-State Circuits, Vol. SC-21, No. 6, December 1986, pages 1026-1034. The complementary positive and negative op amp outputs 12 and 14 are connected to the gates of p-channel MOS transistor P0 and n-channel MOS transistor N0, respectively. The source-drain circuit of P0 is connected between V+ and the Vo terminal, while the drain-source circuit of N0 is connected in series with the source-drain circuit of P0 between the Vo terminal and Vxe2x88x92. While this circuit achieves a substantially rail-to-rail output voltage swing, it is still subject to output phase shifts that can reach 180xc2x0 and thus produce positive feedback oscillation.
The present invention seeks to provide a new voltage amplifier circuit that both inhibits oscillation due to positive feedback, and achieves a rail-to-rail output voltage swing.
This goal is achieved by providing a pair of output stages for the voltage amplifier, with one stage including a blocking impedance and dominating for low values of output voltage at which a rail-to-rail swing is not required, and the second output stage excluding the blocking impedance and dominating for high values of output voltage at which rail-to-rail operation may be desired.
In a preferred implementation of the invention with CMOS output stages, the first output stage provides more AC feedback than the second stage, while the second stage provides DC feedback with an output voltage range approximating rail-to-rail. Further details of a particular implementation include providing the blocking impedance as a resistor connected between the two output stages, with the first and second output stages connected respectively through a feedback capacitor and a feedback resistor to the voltage amplifier. The CMOS transistors of the second stage are larger in size than the first stage transistors, with the size ratio between the transistors sufficient to place the second stage transistors in their linear range when that stage becomes dominant, thereby preventing oscillation. The resistance value of the blocking resistor is also high enough to inhibit oscillation, but small enough so that the second stage transistors are in their linear range when that stage becomes dominant.